The present invention relates to laser beam recorders using a polygonal mirror as a scanning device, and more particularly concerns a novel laser beam recorder arranged to compensate for a tilted facet of the polygonal mirror.
Laser beam recorders have included a laser and optical and mechanical systems arranged to provide a two dimensional photographic image on a photographic film in response to an electrical signal representative of the image. Usually an acousto-optical or electro-optical modulator is used to control or modulate incident flux of the laser beam. The modulated laser beam is then raster scanned across the photosensitive film by the optical and mechanical systems to produce the photographic image. A polygonal mirror is often used to scan one dimension of the desired image. However, expensive precision polygonal mirrors are required to maximize the coincidence of the scan lines produced by each facet of the polygonal mirror since a scan line displacement error is caused if a mirror facet is tilted. The degree of precision required for the mirror facets can be significantly relaxed through the use of corrective optics as disclosed in the IBM Journal of Research and Development, Vol. 21, No. 5, page 481. However, a compact laser beam recorder using fewer and simpler elements than previous designs is needed to compensate for scan line displacement errors caused by polygonal scan mirrors having tilted facets.
Accordingly, it is an important object of this invention to provide a simple, compact laser beam recorder including a polygonal mirror scanner in an optical system adapted to minimize scan line displacement errors associated with rotating polygonal mirror scanners.